Generally, a color liquid crystal display device is treated with a solvent, an acid solution, or an alkali solution during preparation processes thereof, or the surface of the device is partially treated at a high temperature upon formation of a transparent electrode layer through a sputtering process. Occasionally, when the transparent electrode layer is etched to a desired shape, the above device is exposed to an acid solution or an alkali solution under severe conditions. In order to prevent damage to pixels due to heat or chemical material upon such treatment, a protective film, which consists of a thin film resistant to the above treatment, is provided. Further, a display device, such as an in-plane switching (IPS) mode LCD, requires a protective film, having no variation in thickness, to form a uniform liquid crystal layer, and excellent flatness, for maintaining the gaps of cells.
In addition, the protective film should have excellent adhesion to a substrate on which the protective film will be formed and to a layer that will be formed on the protective film, and as well, should have high transparency, heat resistance and light resistance. Moreover, the protective film should be resistant to deterioration, such as coloration or discoloration, for a long period of time, and should exhibit resistance to various chemicals, for example, acids, alkalis and organic solvents. Material for the protective film having such properties is classified into a photocurable material and a thermally curable material, depending on the methods of forming such a film. Known photocurable material is a negative-type photoresist material, disclosed in Korean Patent Laid-open Publication No. 2001-018075 and Japanese Patent Laid-open Publication No. 2001-064337. Known thermally curable material is an epoxy-based thermally curable resin composition, disclosed in Korean Patent Laid-open Publication No. 2001-049485 and Japanese Patent Laid-open Publication No. 2000-239497.
The process of forming the protective film using the photocurable resin composition is as follows. That is, a photocurable resin composition, which is to be processed into a protective film, is applied onto a substrate via an appropriate process, for example, a spin-coating process, prebaked, exposed, developed, and then postbaked, thus, forming a desired film.
On the other hand, since the thermally curable resin composition does not require exposure and development, comprised in the process of forming the film using the photocurable material, neither additional apparatus expense nor a space, such as a clean room are needed for exposure and development. However, the thermally curable resin composition, comprising typically of an epoxy resin and carboxylic acid or carboxylic acid anhydride, causes problems as mentioned in Japanese Patent Laid-open Publication No. 2001-325045, in particular, poor storage stability of the composition due to continuous reaction between the epoxy resin and carboxylic acid or carboxylic acid anhydride serving as a curing agent, and rapid curing of the composition due to the presence of carboxylic acid anhydride. Thus, the thermally curable resin composition has been used in a manner such that the epoxy resin is mixed with the curing agent immediately before use. Recently, although one-part curable resin composition, having reduced reactivity of a curing agent to alleviate the problem of storage stability, has been proposed, it does not sufficiently exhibit satisfactory storage stability. That is, even in the case where the problem of storage stability is alleviated, the reaction between the curing agent such as acid or acid anhydride and the epoxy group occurs continuously as long as each component remains. In practice, a curing process is undesirably caused before a postbaking process, thus decreasing the storage stability of the thermally curable resin composition.
Further, in such a conventional thermally curable resin composition, when the composition ratio of a monomer having an acid group and a monomer having an epoxy group is not controlled in a predetermined range, a cross-linking reaction between the monomers in the copolymer occurs continuously, leading to reduced storage stability of the composition. Continuously, limitations are imposed on synthesis of the copolymer.
Unlike the protective film remaining on the substrate in pattern form through selective exposure and development by use of the photocurable resin composition, the thermally curable resin composition is formed into a film on the whole surface of the substrate without a pattern, and thus, requires higher adhesion to each of the substrate and the layer formed on the protective film than the photocurable resin composition. Specifically, if adhesion is insufficient between the protective film and the color filter layer and between the protective film and the glass substrate, a sealing therebetween may be broken upon injection of a liquid crystal material. Also, upon deposition of ITO, a peeling phenomenon may be caused between the ITO/protective film or protective film/color filter layer. In addition, various thermally curable resin compositions, each having an epoxy resin and a curing agent such as carboxylic acid or carboxylic acid anhydride, suffer because they have poor storage stability, and as well, it is difficult for them to simultaneously assure diverse required properties and adhesion.
In the conventional techniques, although an adhesion adjuvant such as a silane compound may be used to ensure adhesion, it is disadvantageous because the thermally curable resin composition becomes unstable due to the addition of the low-molecular weight component and also various impurities may be generated on pipes upon the process, thus the amount of such an adjuvant is limited.